1. Field of the Invention
The present invention is directed to a device which can indicate when a telephone line is being used, and more particularly, it will indicate when a telephone line is in use when a plurality of telephones are all set up to the same telephone line.
2. Discussion of the Background
When several telephone extensions are each connected to the same telephone line, it is useful to know when one telephone extension is being used so as to avoid picking up and interrupting a telephone call by picking up another telephone extension. In this way, a device for detecting such use of a telephone line may be used when it is desirable to tell if a common line is being used by any of the telephone extensions connected to it.
Several conventional telephone in-use indicators are known in the art. Typically, such telephone in-use indicators detect when a telephone line goes "off-hook", i.e., when one telephone extension is lifted up in preparation of placing a call. When a telephone line goes "off-hook" a low impedance is placed across the telephone line. This low impedance reduces the line voltage in an "on-hook" condition to a lower "off-hook" line voltage and also results in a significant increase in the line current. By detecting either this change in line voltage or this change in line current, it can be detected when a telephone goes "off-hook" and is thus in use. Conventional telephone in-use indicators will detect one of these conditions and thereby provide a visual indication that a telephone line is being used.
However, such conventional telephone in-use indicators suffer from several significant drawbacks. First, the conventional telephone in-use indicators typically use a complicated circuitry system to provide a telephone in-use indication. This results in complicated circuits which are costly. Further, conventional telephone in-use indicators place a load on a telephone line so that only a limited number of such conventional telephone in-use indicators can be applied to a single telephone line.